The present invention relates to anti-fouling marine coatings and more particularly to novel low toxicity anti-fouling agents therefor. One of the earliest needs for performance-oriented coatings was in the marine environment. Early formulations were designed around known toxins, such as copper and mercury compounds. Nineteenth Century marine coatings typically used creosote and natural drying oil formulations bearing the toxins. For ship bottoms, presently, anti-fouling compounds based on copper and tin commonly are incorporated into somewhat water-sensitive binders to afford gradual break-down of the film to permit a sustained release of the "poison". This required self-erosion property necessitates frequent repainting of ship bottoms, depending upon location and severity of exposure conditions.
Today's anti-fouling coatings use two general leaching mechanisms, depending on the type of resin matrix selected, soluble or insoluble. The insoluble-matrix type leaves a resinous skeleton intact as the toxicant particles are removed by dissolving into solution in seawater. This also is called the contact type because it depends upon the toxicant migrating to the surface and entering solution by making contact with seawater. Since the resins are somewhat water-permeable, the toxic particles may diffuse through the semi-permeable coating, and as one particle dissolves, another is exposed to seawater. The contact type contains several times more toxicant than the soluble type. The resulting thicker films of toxicant provide a longer service life to the anti-fouling topcoats. As a general rule, the insoluble-matrix type of paint does not contain an extended pigment, and the geometry of the dry film requires high toxicant loadings (52% to 74% by volume) to ensure the Cu.sub.2 O particles will be in continuous contact with each other. Below the level of cubic packing (52%), the resin will encase the Cu.sub.2 O particles and prevent solution; above the level of hexagonal packing (74%), the coating will be too-resin poor to maintain film integrity. These figures may vary somewhat in actual practice and it is common to adjust the leaching rate and the effective range of toxicant loading (e.g. by the addition of rosin or other natural resins). In common practice, both natural resins and extender pigments are frequently used. When high levels of rosin are used and high erosion might be expected, tougheners such as ester gum, ethyl cellulose, and modified rubbers are added. (Paint Handbook, G. E. Weismantel, McGraw-Hill, New York, N.Y., pp 14-43 and 14-44; and R. J. Dick, Marine Paints, Chapter 14).
With respect to the toxins presently used in marine anti-fouling paints, recent U.S. federal legislation has severely restricted the use of organo-tin anti-fouling agents. Toxicity concerns appear to be a prime motivation behind this recent legislation. Prior proposals include, for example, Japanese Patents 56156202 and 52117425 report the use of a combination of a napthoquinone and a thiuram disulphide in order to obtain anti-fouling activity, while Japanese Patent 63243067 proposes the use of diphenylamines. Despite these proposals, there still is a substantial need in the anti-fouling arena for new, low toxicity anti-fouling coatings, caulks, and the like.